Old-maps.com may seem like an unprofessional source for getting accurate maps of anything with any credibility. However, the services offered in this Atlas for city maps, of not only London (as linked above), is absolutely astounding. This online Atlas allows the user to pull up maps of just about any town in England dating from the late 19th to just before the 21st century. Over a hundred years worth of city maps at your fingertips, making this Atlas a very worthwhile link to check oout.
On a similar note the bigmap blog is a great resource for finding and learning some basic information behind some unique historical maps. For anyone interested in getting better acquainted with the latest trends in historical mapping this is a great place to start.